1. Field of the Invention
This invention relates to a fast acquisition method for obtaining data from a transmission channel, and, in particular, for use in hard disk drives or in burst-mode radio communication systems, comprising the steps of timing acquisition; gain acquisition and word synchronization by means of a preamble that comprises a predetermined bit format, this preamble being followed by N-bit user data words.
2. Description of Related Art
Such a method is known from the documentation of the SSI 32P4904 Read Channel IC, that is disclosed on the pages 5-335 to 5-377 of the 1995 data book of Silicon Systems, Inc., Tustin, Calif., USA.
In hard disk drives, user data is subdivided into fixed-length blocks that are stored in separate sectors. Data blocks are encoded prior to storage, so as to facilitate reliable detection, and are preceded by a predefined preamble that permits the read path to prepare itself for reception of the user data. This preparation is commonly called fast acquisition, and generally comprises the following tasks:
1. locking the read clock to the incoming data stream; this is generally called the timing acquisition; PA0 2. initialization of the gain of the AGC; the so-called gain acquisition; PA0 3. initialization of the adaptive parameters of the equalizer, the equalizer acquisition, this initialization sometimes can be omitted when the parameters of the equalizer are pre-programmed; PA0 4. identification of the boundaries of code-words, the word synchronization; PA0 5. identification of the start of the user data portion of the sector, i.e., the frame synchronization. PA0 1. Simplicity: Word synchronization and acquisition of timing, gain and equalizer occur in one go rather than in multiple steps. Frame synchronization, if provided for in the preamble, takes place at the code-word rate and is simple. PA0 2. Speed: All acquisition steps occur in parallel rather than sequentially. Reliable frame synchronization, if provided for in the preamble, is possible with a very short sync word. PA0 3. Robustness. All acquisition steps are performed in a data-aided fashion and are, hence, immune to decision errors. For the adopted sync-word length, frame synchronization is maximally tolerant to decision errors. Further improvements to error tolerance are simple.
The structure of the preamble reflects the order in which these tasks are dealt with. A typical structure is depicted in FIG. 1, this figure corresponding with FIG. 11 on page 5-348 of the above mentioned documentation of Silicon Systems.
The preamble commences with the pattern ". . . ++-+-++. . . " of period 4T where T is the temporal spacing of recorded bits. This pattern induces a sine-like replay signal that conveys considerable timing and gain information. As is described in the article "Timing recovery for adaptive decision Feedback Equalization of the Magnetic Storage Channel," by W. L. Abbott and J. M. Cioffi, in Proc. GLOBECOM'90, pp. 1794-1799, San Diego, Calif., Dec. 2-5, 1990, or in the article 11A PRML System for Digital Magnetic Recording 11, by R. D. Cideciyan, et al., in IEEE J. Selected Areas Communications, Vol. SAC-10, No. 1, pp. 38-56, January, 1992, prior knowledge about this pattern and signal can be exploited for rapid timing and gain acquisition. Techniques of this type are generally data-aided, i.e., they are tailored to the pattern at hand and cease to function properly when another pattern is recorded. Near the end of the 4T pattern it is, for this reason, necessary for the timing recovery and gain-control loops to switch to a decision-directed tracking mode. In this mode, prior knowledge about the recorded data is replaced by posterior knowledge based on decisions produced by the detector. As long as these decisions are predominantly correct, both loops will properly track the timing and gain of the replay signal. At the end of the 4T portion of the preamble, the AGC is set properly and the timing recovery scheme is phase-locked to the 4T pattern.
The next step in the acquisition method is word synchronization. This step is based on a predefined data pattern that is compared every symbol interval T with a window of detected bits. Word synchronization is established once an adequate match between these two patterns is observed. Modern hard disk drives often use a modulation code with rate R=8/9, so that code-words have a length of 9T. The word-sync pattern in FIG. 1 has a period 9T and is just a repetition of a single predefined code-word. This code word is selected to be able to adequately distinguish the proper code-word alignment from the eight alternative alignments and, moreover, to permit acquisition of a slope equalizer. Such an equalizer is often used in recording systems because the bandwidth of the head/media system is not accurately known beforehand. The equalizer compensates, in essence, for spectral unbalances between low and high frequencies and a suitable training pattern must have at least two spectral components within the pass-band of the head/media system. The 4T pattern cannot qualify because it has only one such component.
At the end of the 9T pattern, the only task left is frame synchronization. In FIG. 1 this is accomplished with a single predefined code-word that differs sufficiently from the preceding 9T words. The frame sync check can be based on the input of the modulation decoder, but also -at some loss of Hamming distance- on its byte-wise output. It needs only be performed every byte-interval, i.e., every 9T seconds, and is, as such, much less computationally intensive than the word sync check, which is performed at the bit rate 1/T.
It should be mentioned that there are many variants of the above method. For example, the tasks of word and frame synchronization can be combined at the cost of a longer sync word and more complicated synchronization hardware, as, for example, is described in the above mentioned article of W. L. Abbott and J. F. Cioffi. The equalizer training pattern can then follow the sync word and is no longer restricted in period and length. This is important if the equalizer has multiple "knobs". In some cases, the equalizer has too many knobs to permit training prior to every data read operation. In these cases, equalizer coefficients are normally trained once during disk drive manufacturing, and are pre-loaded ever after. This frees the preamble from a pattern for equalizer training. The other 4 tasks are invariably encountered in modern channel ICs. For the sake of completeness, it is worth mentioning that frame synchronization was accomplished outside the channel IC in past generations of hard disc drives, notably for channel ICs with (1, 7) or (2, 7) run-length-limited code.
To a larger or smaller extent, existing acquisition methods all share the following characteristics and disadvantages:
1. They are largely serial in nature, i.e., the various tasks are largely accomplished consecutively. This complicates hardware and lengthens acquisition-times.
2. Several of the involved steps (e.g., equalizer acquisition, word and frame synchronization) occur in a decision-directed mode, and are sensitive to decision errors.